Nit: in dual_inner_prod_sse, why not do both horizontal sums at the same
time? As in:
xsum1 = _mm_add_ps(_mm_movelh_ps(xsum1, xsum2),
_mm_movehl_ps(xsum2, xsum1));
xsum1 = _mm_add_ps(xsum1, _mm_shuffle_ps(xsum1, xsum1, 0xf5));
_mm_store_ss(xy1, xsum1);
_mm_store_ss(xy2, _mm_movehl_ps(xsum1, xsum1));
--John